1. Field of the Invention
The present invention relates to cathode assemblies for use in Hall-Heroult aluminum reduction cells, more particularly, to cathode assemblies having a plurality of interlocking wettable ceramic tiles covering the cathode blocks.
2. Prior Art
Aluminum is commonly manufactured by a smelting process in an electrolytic cell of the established Hall-Heroult design. A conventional Hall-Heroult electrolytic cell includes a cell defining a chamber housing carbonaceous anodes. The anodes are suspended in a bath of electrolytic fluid containing alumina and other materials. Electric current is supplied to the anodes to provide a source of electrons for reducing alumina to aluminum that accumulates as a molten aluminum pad. The molten aluminum pad forms a liquid metal cathode. A cathode assembly is positioned in the bottom of the chamber and completes the cathodic portion of the cell. The cathode assembly includes cathode blocks having an upper surface, which supports the molten aluminum pad. Collector bars are received within a lower portion of the cathode blocks and are connected via a bus bar to a current supply in a conventional manner to complete the circuit.
These electrolytic cells are typically operated at high temperatures (about 940 to 980° C.) which, when combined with the corrosive nature of electrolytes creates a harsh environment. Cathode blocks have historically been formed from a mixture of anthrocite and pitch binder and exhibit relatively high electrical resistivity, high sodium swelling, low thermal shock resistance, and high abrasion resistance. As aluminum producers seek to increase productivity, the operating amperages for such cells have been increased. Hence the need for reduced power losses in the smelting process has increased. One limitation in the operation of an electrolytic cell is the distance between the lower surface of the anode and the upper surface of the liquid metal cathode. Conventionally, this distance has been about 4 to about 5 centimeters. It is well-established that substantial savings in the electrical energy required for the operation of the cell could be achieved by reducing the distance between the anode and the cathode. Reduction of the anode to cathode distance in conventional electrolytic reduction cells has been limited by the strong magnetic forces in the horizontal plane as a result of the interaction of horizontal current components in the molten metal with strong magnetic fields existing within the cell. The magnetic forces acting on the molten metal lead to an intermittent shorting between the anodes and the molten metal cathode when the anode to cathode distance is reduced below the conventional 4 to 5 cm.
More recently, it has been recognized that these difficulties may be obviated by covering the cathode block with individual packing elements with a surface which is resistant to attack but yet is wettable by the molten metal, but not wettable by the molten electrolyte thereby using the interfacial tension forces of the molten metal/electrolyte interface to restrain entry of the molten electrolyte into the bed of packing elements. Such a system is disclosed in U.S. Pat. No. 4,443,313, incorporated herein by reference, which discloses a tightly packed monolayer of loose elements formed from materials, such as TiB2, in various geometric shapes. A significant drawback to the system disclosed therein is the moveability of the packing elements, particularly in the vertical direction.
Accordingly, a need remains for an electrolytic cell which may be operated with a reduced anode/cathode distance by including a surface on the cathode block which is wettable by the molten metal yet is not subject to shifting during operation of the cell.